How do string quartet covers differ from the original tracks?

The corpus I will delve into is Vitamin String Quartet’s covers of popular songs. As their discography is quite large, I cannot analyze all their music, so I will focus on a random selection of 104 tracks. These tracks have been added to a playlist, while their original versions have been added to a seperate playlist. This way, the two comparisons are only of the tracks and their originals.

I chose this corpus because I love string quartet music. I play violin and cello myself and thus am personally attached to the type of instruments used, but I also think having a group of four instruments makes it possible to hear each instrument individually whilst being able to appreciate how they sound together. The layers of the music can each be distinctly heard. Covering popular music gives a creative twist to the original tracks, while showing people that instruments generally associated with classical music are very versatile and can even motivate people to also go and listen to classical music.

The most natural comparison points are the original versions of the covered songs. The most obvious differences will likely be acousticness and instrumentalness. I imagine danceability, loudness and energy could differ as well, but they could be higher in either one. I am curious to see whether fundamental parts of a track like the key and tempo will be altered in the covers.

In using a specific artist for analyzing string quartet covers, there will likely be more coherency between the covers than if I were to take several string quartets. In this sense, it is not entirely representative of all string quartet covers. While the orginal tracks the covers are based on span across several genres, much of it is popular music and therefore it will largely be a comparison to pop music. In this sense, it is not representative of all genres.

Some tracks that I imagine will be atypical are “Little Lion Man” and “Mr. Brightside”. The way they chose to change the sound of the instruments is quite far from the originals, where in most cases the general melodies of the track remain the same. Some tracks I think will be typical are “Weird Science” and “Bad Guy”. While the instrumentalization is, naturally, changed to only four string instruments, every melody in the originals is clearly recognizable in the covers.

Valence, Energy and Loudness in String Quartet Covers compared to their Original Versions

Comparing valence, energy and loudness shows us how having only a string quartet as the performers of the piece changes the mood of a song. The graphs below show us this difference, making it clear that especially the energy of a song is quite different in a string quartet version. The size of the points represents the track’s loudness, and while there is not a very large difference, it also seems most string quartet versions are not as loud. For these graphs, I used a playlist I made of 104 covers by Vitamin String Quartet, and a playlist I made of the original versions of these 104 songs. When hovering over the graphs, you can see the track names and the exact value of valence, energy and loudness per track.

Chromagrams: Are string quartet versions lower pitched?

This comparison of the chromagrams of the original and the string quartet rendition of ‘Wonderwall’ already show that the pitches in both versions are different. As a chromagram captures harmonic and melodic characteristics, you would expect a cover track to be similar. I chose this track from my corpus because the duration of the two versions was so close that I thought they would be easily comparable. From these comparisons, however, it seems that the string quartet version is generally lower pitched than the original. On the next tab, the two will be combined to show their alignment with the dynamic time-warping technique.

Dynamic Time Warping: The alignment of Oasis’s ‘Wonderwall’ versus its String Quartet cover

With Dynamic Time Warping, we can compare the pitches of the two versions in a single visualisation. If the two tracks have the same pitches, but are performed by two different artist, we can see a diagonal line in this visualisation. In this one, there is no line and it is very difficult to see the similarity of pitches in the two versions of ‘Wonderwall’. This leads us to conclude that while one is copying the melodies of the other, they do not align well. Despite this being the clearest option of the different normalisation and distance combinations, the chromagram is still quite unclear.

Now that we have compared string quartet covers to their originals, let’s compare a cover to itself.

These self-similarity matrices compare the chroma (pitches) and timbre (character) of Vitamin String Quartet’s rendition of the Oingo Boingo classic ‘Dead Man’s Party’. The visualisation on the left represents chroma, showing us at which points the same pitches appear in the track. The one on the right shows us the same thing for timbre, which is based on several factors like harmonic structure, frequency and intensity. Timbre gives us the “tone color” or “tone quality” of an instrument, which distinguishes it from other instruments.

In these visualisations, we see several diagonal lines. They are most clear in the one comparing chroma. These lines represent paths that define similar segments. When you listen to the track while looking at the matrix, you can see that the first small diagonal paths are where short melodies repeat, while the longer, very clear ones are a repeat of a longer melody segment. The bright, perpendicular lines show a sudden new section. This is also visible in the track, as this is the point where an entirely new melody starts.

In the timbre matrix, we cannot see too many clear paths. We do, however, see bright lines representing novelty, and a checkerboard pattern which shows us homogeneity. As this is a string quartet cover, the instrumentation is all the same four instruments, and they are all string instruments. This will likely be the cause of the checkerboard pattern. The bright lines could represent the points where the certain instruments either come in, or stop playing.

Is the key of a piece changed when it is rewritten for a string quartet?

Musical key describes the scale on which a song is based, meaning that most of the notes in a song will come from the scale of that key.In this bar plot, we can see that for the different keys, the number of songs in the playlist of Vitamin String Quartet covers (Vitamin for CompMusic) is not the same as the number of originals (Originals for CompMusic) in that same key. As the Vitamin String Quartet playlist only contains the covers of the songs on the Originals playlist, and no others, this means that the keys of several songs have to have been changed when transitioning to their string quartet version.

How does the tempo change when adapting a piece for a string quartet?

As I was examining my corpus’s tempo, I saw that there are no clear outliers, and both the Vitamin String Quartet Covers playlist and the Originals Playlist stayed within roughly the same range. Upon closer look, however, I found that the highest and lowest tempo songs did not match across playlist. For example, the highest tempo song in the string quartet versions was ‘Little Black Submarines’. This one was not particularly high ranking in the Originals. This is why I thought it would be interesting to compare tempograms of these two versions.

In the original ‘Little Black Submarines’ by The Black Keys, you can see that there is a very obvious sound change at around 130 seconds. There is a short pause and then a distorted guitar. This is clearly visible in the tempogram, where you can see it is having difficulty determining the tempo there, and it therefore looks like a vague gap in the piece. After this first change, the song goes back to the melody it had before, but with different instrumentation and it sounds like the tempo is slightly higher. This change is also visible in the tempogram, where you can see the lines shift so that they do not align perfectly with the part before 130 seconds.

According to Spotify, the tempo of the Vitamin String Quartet version of ‘Little Black Submarines’ is a little over double the BPM of the original. When listening to both, I can imagine it, as the string quartet version has strings playing in the background that make it sounds faster than the very basic instrumentation of this part of the original. Something that is visible in both tempograms is the change when the song turns to more of a rock song. The tempogram for Vitamin String Quartet also shows this sudden change at around 130 seconds.

Can a classifier determine the difference between string quartet covers and their original tracks?

In this confusion matrix, we look at a classifier’s predictions of which song will fit which playlist, compared to which playlist it is actually in. For this matrix, I fed the classifier both my Vitamin String Quartet covers playlist and my playlist of the original versions in question. The Spotify API features used to train the classifier are danceability, energy, valence, loudness, speechiness, acousticness, instrumentalness, liveness, tempo, duration and key.

I experimented with leaving particular audio features out. For some cases, valence for example, this did lead to more accuracy, but it would not be representative to leave important features out.

For the Originals playlist, the precision was 0.750, while the recall was 0.9. For the Vitamin String Quartet playlist the precision was 0.875, while the recall was 0.7. While a human would immediately be able to tell whether or not the song was only played by string instruments, the classifier still did a decent job.